1. Field of the Invention
The present invention relates generally to electrical stimulation systems, and more particularly, to the control of charge imbalances in electrical stimulation systems.
2. Related Art
Electronic devices implanted within the body in order to stimulate nerve tissue (e.g. cochlear implants) for perceptual or functional purposes generally use platinum electrodes as the interface between the electronics and the body tissue. In general terms, such electrodes are selectively driven with a current in order to evoke a perception (for example sound) or a function (for example a limb movement) in the user. In application, a stimulating current is applied to the implanted electrodes. This current passes through the implant recipient's tissue and the nerve cell, and returns to the implant. At the surface of the electrodes, chemical reactions may take place, changing the electron current in the electronics to an ion current in the tissue. Further, a charge remains on the electrode surface, causing an increase in voltage across the electrode-tissue interface. Under normal operation of the interface, these chemical reactions are reversible when the current direction is changed, leaving a neutral interface.
It is usual for the stimuli to be structured as biphasic pulses, in such a way that there is no net charge delivered to the tissue. If, however, the current is allowed to flow in one direction for too long, toxic products can escape the interface and damage or destroy the surrounding tissue. Likewise, if the voltage across the interface is allowed to remain elevated for too long, toxic species are irreversibly generated at the interface. To ensure that stimulation is safe, and that no toxic species escape the interface, it must be ensured that the DC and low-frequency (LF) states of the electrodes, i.e. the DC/LF interface voltages and the DC/LF interface currents, remain within certain bounds. The usual target values are a factor of hundreds of milli-volts, or some tens of nano-amperes (for typical cochlear implant electrode areas of about 0.25 mm2) The United State's Food and Drug Administration (FDA) requires that the magnitude of the current through an electrode be below 100 nA measured over any 1 milli-second period.
The use of charge-neutral pulses ensures, in principle, that the FDA requirement for the DC/LF current is met. In practice, however there will be a small error in the generated stimulation current. This requires a second measure to be taken to ensure low levels of DC/LF current are maintained at all times. This is particularly an issue when high stimulation rates and high current levels are used. Further, if the stimulation current source goes out of compliance, then significant charge errors can occur. A number of approaches are currently employed to control the interface voltage and current.
One approach is to use DC blocking capacitors for each electrode to ensure zero DC currents through the electrodes. This DC blocking capacitor is disposed in the stimulation current path. A capacitor may also be disposed in the stimulation current path of the monopolar return electrode in implementations employing monopolar stimulation. In order for this approach to be effective, it is necessary to provide a capacitor with relatively high capacitance, in the hundreds of nano-farad range, for each electrode. With current capacitor technology, this cannot be fabricated in an integrated circuit, and so discrete components are used, which may increase the required space for the implant. This type of approach is discussed in, for example, U.S. Pat. No. 5,324,316 to Schulman et al, U.S. Pat. No. 6,600,955 to Zierhofer et al, and U.S. Pat. No. 6,219,580 to Faltys et al.
Another approach is to use periodic short-circuiting of all electrodes to ensure that the DC/LF electrode voltage does not drift out of the safe window. This typically employs using a shorting switch for each electrode, and periodically closing the switches thereby connecting all electrodes to ground. In some implementations (e.g., monpolar stimulation), a series capacitor is used in the return electrode only. This approach allows for up-scaling of the number of electrodes. However, shorting all the electrodes requires the stimulation protocol to include an inactive period when no stimulation takes place. This approach is discussed in European Patent No. 0,241,101 to Cochlear Limited.
Another approach is to measure the differential voltage between electrodes during a dead period and adjust the duration or amplitude of the applied stimuli to compensate for the charge error. This approach is disclosed in U.S. Pat. No. 5,674,264 to Cochlear Limited.